<!DOCTYPE html>
<svg width="960" height="500">
  <path transform="translate(180,150)scale(2,2)" fill="none" stroke="black" stroke-width="1.5"></path>
</svg>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script>

var d1 = "M0,0c100,0 0,100 100,100c100,0 0,-100 100,-100",
    d0 = "M0,0c100,0 0,100 100,100c100,0 0,-100 100,-100c100,0 0,100 100,100";

d3.select("path")
    .attr("d", d0)
  .transition()
    .duration(8000)
  	.attrTween("d",pathTween(d1,2))


var path = d3.select("svg").append("path").attr("d",d0).style("fill","none").style("stroke","black")

function pathTween(d1, precision) {

  return function() {
  
    var path0 = this,
        path1 = path0.cloneNode(),
        n0 = path0.getTotalLength(),
        n1 = (path1.setAttribute("d", d1), path1).getTotalLength();
       

    // Uniform sampling of distance based on specified precision.
    var distances = [0], i = 0, dt = precision / Math.max(n0, n1);
    while ((i += dt) < 1) distances.push(i);
    distances.push(1);

    // Compute point-interpolators at each distance.
    var points = distances.map(function(t) {
      var p0 = path0.getPointAtLength(t * n0),
          p1 = path1.getPointAtLength(t * n1);
          var a = d3.interpolate([p0.x,p0.y],[p1.x,p1.y]);
         
      return d3.interpolate([p0.x, p0.y], [p1.x, p1.y]);
    });
	 
    return function(t) {
    	console.debug(t)
      return t < 1 ? "M" + points.map(function(p) { return p(t); }).join("L") : d1;
    };
  };
}

</script>